Add timestamps for pte generation in CI

🚀 The feature, motivation and pitch

Regression tracking for aot export times: add some timestamps to indicate how long the llama export took:
https://github.com/pytorch/executorch/blob/d67fb523d9ed6f7ddd09e80782bfe4e369fc99c4/.ci/scripts/test_llama.sh#L268

And fail if the export time is over some threshold. For now, we can set the threshold to be the time it takes to export the longest-running configuration.

The breakdown can look something like this:

1. Add timestamps for the export command in CI.
2. Observe the time it takes for each llama test in CI runs. Expect different export commands to show different export times.
3. Add a fail, if the export time exceeds the largest time observed in (2), with some buffer; I guess we expect some variability with each run.
4. Optional add-on: make (3) more fine-grained, and targeted per-command.

Alternatives

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Additional context

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RFC (Optional)

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